1. Technical Field of the Invention
This invention relates generally to a steering system for a vehicle having steerable front and rear road wheels and methods for operating the steering system, more particularly, a hydraulic steering system for a vehicle having mechanical controls for steerable front and rear road wheels.
2. Description of the Prior Art
There are vehicle steering systems for steerable front and rear road wheels in common use. These steering systems are sometimes found on tow tractors for providing a three-mode, four-wheel steer vehicle. Such systems include a servo valve to control the turning of the steerable rear road wheels. In other words, the servo valve controls the hydraulics to the rear road wheels as a result of signals received for the turning from a microprocessor. The microprocessor receives both input electronic signals from a front road wheel sensor and feedback electronic signals from a rear road wheel sensor. The microprocessor compares both signals and energizes the servo valve to turn the rear road wheels either left or right as required to synchronize with the turning position of the front wheels.
This electronic steering system is operated full-time and is completely automatic. A three-position mode selector switch is provided in the operator's compartment to allow the operator to select one of the three modes of operation: (1) front wheels turn only, rear wheel straight ahead, (2) front and rear wheels turn the same angular direction (crab) and (3) the front and rear wheels turn at angles opposite to one another (coordinated).
The front wheels turning only mode for the electronic system is achieved by wiring the mode selector switch to a constant resistance to simulate the resistance of the front wheel sensor to a straight-ahead position. This electronic signal is sent continuously to the microprocessor with the microprocessor in turn sending current to the servo valve directing the rear wheels to be aimed straight ahead. The above coordinated mode is achieved by simply reversing polarity of the front wheel sensor.
These microprocessor based electric-over-hydraulic four-wheel steering systems use hydraulic pumps to position all four wheels. In every case, the front wheels are positioned hydraulically by using a hydraulic steering valve connected to the steering wheel to direct fluid to the desired front wheel cylinders.
A problem with this system occurs when moisture or water accumulates on any part of the circuit involving the wheel sensors. The rear road wheels may, without any input from the driver, go hard over left or right in a failure mode resulting from this moisture. This, of course, is quite dangerous for an operator or for the payload being towed by the tow tractor.
Another problem with these electronic systems occurs during cold-weather operation. The cold weather causes changes in electron flow producing undesired relative angular positioning of the rear and front road wheels. Also, steam cleaning of the microprocessor and sensors, degradation due to corrosion, broken wires, shorts, mechanical forces acting on the wheel sensor causing a change in resistance and mechanical wear-out of wiper arm on the wheel sensor are other serious problems experienced with this electric-over-hydraulic system.
Additionally, this electronic steering system requires trained electronic technicians with high precision electronic equipment. Since these electronic system failures are usually not visually detectable and are not intuitively obvious, field maintenance is more of a problem.
Additionally, the wheel sensors are located in the wheel wells on axle king pins, which makes adjustments to the wheel sensors difficult when the road wheel is on the vehicle. It has also been found that touching wires to ground when hooking up components "zapped" sensitive parts of microprocessor.